From: Ben Clewett
Date: April 8 2008 4:36pm
Subject: Re: Help with ORDER BY using two colomns [ solved thankyou :) ]
List-Archive: http://lists.mysql.com/mysql/212152
Message-Id: <47FB9F19.1090004@clewett.org.uk>
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Richard,
The query I gave you required the column 'o' to be calculated for each
row at the time of gathering the data. When all rows have been
gathered, the data will be stored and sorted in a temporary table. This
temporary table will be in memory or on disk depending on the setting of
the variable 'tmp_table_size'. Then the result will be pruned to the
LIMIT and sent.
You can see that the LIMIT does not help, MySql needs to know what the
top rows will be, before it discards the bottom ones. It can't know
this until all data has been gathered and sorted.
This is all quite a lot of work for your MySql. Hence it is not
regarded as scalable.
------------
Alternatively, store the order permanently on the table as a new field
and give it an index:
ALTER TABLE .... ADD o INT NOT NULL,
ADD KEY(o);
Now the job of MySql is easy, the query just follows the index. No
temporary tables, no scanning all rows and no sorting before sending,
and the LIMIT now works as you would hope. This is highly scalable.
But you have to maintain the ordering field. If your brave, do it with
a trigger :)
Ben
Richard wrote:
> Thanks,
>
> This is for the unanswered list of questions, so the output list (not
> the list stored in the mysql database) should never go over 100.
>
> by scalable, do you mean alot of ressources being used or a long wait
> for the answer? Because I belive I Could just use a simple limit if I
> needed to have a limited number of results on one page.
>
> Every time a question is answered the update date will change, and the
> status could also change. So I don't see how to easily do this by
> creating another table.
>
> Ben Clewett a écrit :
>> Richard,
>>
>> No problem, glad it works. But note: this is not scalable. If you
>> have more than a few hundred rows, you may want to think about a
>> better solution, like storing the order field permanetly and giving it
>> an index :)
>>
>> Ben
>>
>> Richard wrote:
>>> Thanks, it works like a charm :)
>>>
>>> Ben Clewett a écrit :
>>>> A modification to my last email, try:
>>>>
>>>> SELECT
>>>> *, IF(update != '', update + 10, date) AS o
>>>> FROM
>>>> my_table
>>>> ORDER BY o DESC;
>>>>
>>>> +-----+------+--------+------+
>>>> | num | date | update | o |
>>>> +-----+------+--------+------+
>>>> | 5 | 40 | 90 | 100 |
>>>> | 2 | 10 | 60 | 70 |
>>>> | 6 | 50 | | 50 |
>>>> | 4 | 30 | | 30 |
>>>> | 3 | 20 | | 20 |
>>>> | 1 | 1 | | 1 |
>>>> +-----+------+--------+------+
>>>>
>>>>
>>>> Richard wrote:
>>>>> Thanks,
>>>>>
>>>>> I think that your solution will be sufficient for my needs, however
>>>>> I would still like to know for my personal knowledge how to manage
>>>>> correctly this kind of need.
>>>>>
>>>>> And to make it more complicated I've just rearlised that there is
>>>>> another element to take into account, I would need to add 10 days
>>>>> to the update dates so they would place themselves in the correct
>>>>> position.
>>>>>
>>>>> This is how I need the system to work :
>>>>>
>>>>> Any new requests (without an update value) are ordered by date
>>>>> I want to be able to answer these requests (adding a time stamp to
>>>>> the update field and if the customer does not answer within 10
>>>>> days, to re insert them into the list.
>>>>>
>>>>> But as the update timestamp will be 10 days old, I would like to
>>>>> add 10 days to the update while inserting them to the list (not
>>>>> changing the actual value inserted in the database just add 10 days
>>>>> during the reordering process.). I hope my explanation in
>>>>> understadable ...
>>>>>
>>>>> :)
>>>>>
>>>>> Rafael Barbolo Lopes a écrit :
>>>>>> Can't you do Something like:
>>>>>>
>>>>>> ORDER BY (update,date)
>>>>>>
>>>>>> The major column of ordering would be update and the second date.
>>>>>>
>>>>>> I'm not sure about this "solution"
>>>>>>
>>>>>> On Tue, Apr 8, 2008 at 8:54 AM, Richard >>>>> > wrote:
>>>>>>
>>>>>> Hello I've tried the following with mysql 4.1.11
>>>>>>
>>>>>> SELECT * FROM quick_contact WHERE (`status` = '0') OR (`status` =
>>>>>> '2' AND `update` < '".(time()-864000)."') CASE WHEN `update` = ''
>>>>>> THEN ORDER BY `date` DESC ELSE ORDER BY `update` DESC END CASE;
>>>>>>
>>>>>> It does not work but, is it my code that is wrong or is it
>>>>>> just that
>>>>>> case does not work with mysql 4.1.11 ?
>>>>>>
>>>>>> Thanks :)
>>>>>>
>>>>>> Kristian Myllymäki a écrit :
>>>>>>
>>>>>> mysql version?
>>>>>>
>>>>>> http://dev.mysql.com/doc/refman/5.0/en/case-statement.html
>>>>>>
>>>>>> order by case when updated is not null then updated else
>>>>>> created
>>>>>> end desc;
>>>>>>
>>>>>> /Kristian
>>>>>>
>>>>>> On Tue, Apr 8, 2008 at 1:04 PM, Richard >>>>> > wrote:
>>>>>>
>>>>>> Hello,
>>>>>> I've got a table which containes two date colomns.
>>>>>> The first one is called `date` and the second `update`
>>>>>> In the first one I put the ticket creation date, and on
>>>>>> update I add or
>>>>>> change the update value.
>>>>>> So the update colomn does not contain a value until the
>>>>>> first update has
>>>>>> been done.
>>>>>> I would like to order the tickets by their last update
>>>>>> value. And if this
>>>>>> value does not exist use the date value.
>>>>>>
>>>>>> at the moment I use this :
>>>>>>
>>>>>> ORDER BY `date` DESC"
>>>>>> and I would like to replace it by something like this :
>>>>>>
>>>>>> ORDER (IF `update`!= '' BY UPDATE ELSE BY DATE)
>>>>>>
>>>>>> I know this code is completly wrong, just to try and
>>>>>> show
>>>>>> you what I need
>>>>>> ...
>>>>>>
>>>>>> Here is an example of what I want to achieve
>>>>>>
>>>>>> num | date | update
>>>>>> -------------------------------------------
>>>>>> 1 | 1 |
>>>>>> 2 | 10 | 60
>>>>>> 3 | 20 |
>>>>>> 4 | 30 |
>>>>>> 5 | 40 | 90
>>>>>> 6 | 50 |
>>>>>>
>>>>>> The required result would be :
>>>>>>
>>>>>> num | date | update
>>>>>> -------------------------------------------
>>>>>> 5 | 40 | 90
>>>>>> 2 | 10 | 60
>>>>>> 6 | 50 |
>>>>>> 4 | 30 |
>>>>>> 3 | 20 |
>>>>>> 1 | 1 |
>>>>>>
>>>>>> Thanks in advance :)
>>>>>>
>>>>>> --
>>>>>> MySQL General Mailing List
>>>>>> For list archives: http://lists.mysql.com/mysql
>>>>>> To unsubscribe:
>>>>>> http://lists.mysql.com/mysql?unsub=kristian@stripped
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> -- MySQL General Mailing List
>>>>>> For list archives: http://lists.mysql.com/mysql
>>>>>> To unsubscribe:
>>>>>> http://lists.mysql.com/mysql?unsub=barbolo@stripped
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> .:: Rafael Barbolo Lopes ::.
>>>>>> http://barbolo.polinvencao.com/
>>>>>
>>>>>
>>>>
>>>
>>>
>
>